• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 290 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5116 - Switch Node under Load

Larry Simpson
Prodigy 50 points
Other Parts Discussed in Thread: LM5116

Greetings,

I'm working on a PSU that uses the LM5116 Synchronous Buck Controller.  As I increase the load on the supply, the amplitude of the voltage on the switch node decrease (duty cycle increases).  I have discussed this issue with a few of my peers and we can't seem to figure out what's causing the reduction of voltage at the switch node.  So, my questions are:

1.  Is this normal?

2. What could be potential causes of this behavior?

Any assistance is appreciated.

-Larry

  • 0
    TI__Guru**** 191445 points

    That is common for buck converters.  As teh current increases the IR drop across the high side FET increases and the peak output voltage at the switch node decreases as you observe.  The output voltage is teh time average of the switch node voltage, so the regulator widens the duty cycle to maintain output voltage regulation.

    So this is normal operation.

  • 0 in reply to JohnTucker
    Prodigy 50 points

    John,

    Thank you for your response.  However, I should have stated some numbers in my original post.  The converter is designed for an input voltage of 50-70V and an output of 12V @ 2A.  When I apply a load greater than 200mA I see the switch node voltage begin to drop.  At 1A load, the switch node voltage is approximately 75% of the input voltage.  At 1.2A, the switch node displays erratic behavior. I understand that there will be some IR drop across the high side MOSFET but I would expect this to be on the order of hundreds of mV.  I appreciate the help.

    -Larry

  • 0 in reply to Larry Simpson
    TI__Guru**** 191445 points

    Can you post your schematic and some waveforms that illustrate your issue?